This invention relates to a liquid filter of the type in which an active portion of an elongated indexable filter web extends downwardly into a body of dirty liquid in a tank and across an opening in the submerged underside of a hollow box defining a vacuum chamber. Filtering of the liquid takes place as the liquid is sucked through the web and into the vacuum chamber. When the pores of the active portion of the filter web become so clogged as to substantially impair the flow rate, the filtering action is interrupted and the web is advanced endwise to bring a fresh length thereof into the filtering position.
A typical filter of the foregoing type is disclosed in Casson U.S. Pat. No. 3,305,094 in which the filter web is stretched between a supply roll and a pair of take-up rolls and is indexed by rotating one of the take-up rolls. In another type of filter, the filter web is backed by a flexible conveyor which is adapted to be advanced to effect indexing of the web. In these filters, difficulty is encountered in initially threading a new web into position beneath the vacuum box and in maintaining good control over the web during indexing.